The synthesis of heat-shock proteins (hsp) in cells exposed to stress is one of the most highly conserved genetic regulatory systems known and has been observed in organisms as diverse as bacteria and man. The major hsp are nearly 45% homologous over this evolutionary range. Although hsp have been studied intensively, little is known about their functions. We have shown that one of the most abundant proteins (P70) in mouse spermatogenic cells is related closely to hsp70, the major heat- inducible protein. When hsp70 is heat-induced in spermatogenic cells, it migrates to the same location as P70 on two-dimensional polyacrylamide gels. P70 shares antigenic and ATP-binding properties with products of the hsp70 gene family, but the peptide map of P70 differs from that of hsp70. This suggests that P70 is a novel hsp of spermatogenic cells which is synthesized in association with germ cell differentiation. We have also shown that P70 is synthesized normally in spermatogenic cells, along with the other members of this protein family. This is consistent with the recent observations that the expression of heat-shock genes may be regulated developmentally. Future studies will examine the stage-specific synthesis of hsp in germ cells. Information gained from these studies should be useful in determining the physiological role of hsp and in better understanding the effects of environmental agents on male reproduction.